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A time series signal. It should be sorted by x. There is implicit
linear interpolation between each sample, so a discontinuity requires two
samples with the same x.

TODO This is different from karya's Signal, which doesn't interpolate. The
reason is that I didn't want to have doubled samples for common
discontinuities. But I need interpolation here because it's audio-level and
otherwise I get clicks, while karya emits MIDI which relies on the
synthesizer doing the same interpolation, just with a hardcoded latency.
But then there is a problem, how do I turn a karya Signal into a synth
signal? Maybe I should switch karya back to interpolation? Or maybe I just
have to be careful to make karya emit doubled samples for explicit
discontinuities. Or I could emulate MIDI and do a fixed latency
interpolation.